linux-kernel - [PATCH] procfs: Return -ESRCH on /proc/N/fd/* when PID N doesn't exist

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Message-ID: <20150214204452.GA1777871@mail.thefacebook.com>
Date:	Sat, 14 Feb 2015 12:44:52 -0800
From:	Calvin Owens <calvinowens@...com>
To:	Cyrill Gorcunov <gorcunov@...il.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	Alexey Dobriyan <adobriyan@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Peter Feiner <pfeiner@...gle.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>,
	<linux-kernel@...r.kernel.org>, <kernel-team@...com>,
	Pavel Emelyanov <xemul@...nvz.org>
Subject: [PATCH] procfs: Return -ESRCH on /proc/N/fd/* when PID N doesn't
 exist

Currently, readlink() and follow_link() for the symbolic links in
/proc/<pid>/fd/* will return -EACCES in the case where looking up the
task finds that it does not exist.

This patch inlines the logic from proc_fd_access_allowed() into these
two functions such that they will return -ESRCH if the lookup in /proc
races with the task exiting. Since those were the only two callers of
that helper function, it also removes it.

Signed-off-by: Calvin Owens <calvinowens@...com>
---
 fs/proc/base.c | 47 ++++++++++++++++++++++++++---------------------
 1 file changed, 26 insertions(+), 21 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 3f3d7ae..308fcbd 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -485,23 +485,6 @@ static int proc_pid_syscall(struct seq_file *m, struct pid_namespace *ns,
 /*                       Here the fs part begins                        */
 /************************************************************************/
 
-/* permission checks */
-static int proc_fd_access_allowed(struct inode *inode)
-{
-	struct task_struct *task;
-	int allowed = 0;
-	/* Allow access to a task's file descriptors if it is us or we
-	 * may use ptrace attach to the process and find out that
-	 * information.
-	 */
-	task = get_proc_task(inode);
-	if (task) {
-		allowed = ptrace_may_access(task, PTRACE_MODE_READ);
-		put_task_struct(task);
-	}
-	return allowed;
-}
-
 int proc_setattr(struct dentry *dentry, struct iattr *attr)
 {
 	int error;
@@ -1375,10 +1358,21 @@ static void *proc_pid_follow_link(struct dentry *dentry, struct nameidata *nd)
 {
 	struct inode *inode = dentry->d_inode;
 	struct path path;
-	int error = -EACCES;
+	int error = -ESRCH;
+	int allowed = 0;
+	struct task_struct *task;
 
 	/* Are we allowed to snoop on the tasks file descriptors? */
-	if (!proc_fd_access_allowed(inode))
+	task = get_proc_task(inode);
+	if (task) {
+		allowed = ptrace_may_access(task, PTRACE_MODE_READ);
+		put_task_struct(task);
+	} else {
+		goto out;
+	}
+
+	error = -EACCES;
+	if (!allowed)
 		goto out;
 
 	error = PROC_I(inode)->op.proc_get_link(dentry, &path);
@@ -1417,12 +1411,23 @@ static int do_proc_readlink(struct path *path, char __user *buffer, int buflen)
 
 static int proc_pid_readlink(struct dentry * dentry, char __user * buffer, int buflen)
 {
-	int error = -EACCES;
+	int error = -ESRCH;
+	int allowed = 0;
+	struct task_struct *task;
 	struct inode *inode = dentry->d_inode;
 	struct path path;
 
 	/* Are we allowed to snoop on the tasks file descriptors? */
-	if (!proc_fd_access_allowed(inode))
+	task = get_proc_task(inode);
+	if (task) {
+		allowed = ptrace_may_access(task, PTRACE_MODE_READ);
+		put_task_struct(task);
+	} else {
+		goto out;
+	}
+
+	error = -EACCES;
+	if (!allowed)
 		goto out;
 
 	error = PROC_I(inode)->op.proc_get_link(dentry, &path);
-- 
1.8.1

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